One of the problems encountered with laser markable white pigment compositions used as coating on an insulating cover of a wire is that of making the coating sufficiently thin to avoid undesirable increase weight per unit length of the wire whilst at the same time being sufficiently optically dense to obscure any darker coloration present in the underlaying layer of the wire. The optical density of the coating maybe increased by increase of thickness but this not only undesirably increases the weight per unit length of the wire but can result in increased production difficulties, micro-cracking and increased expense due to the requirement for more applications of the coating to build up the required thickness.
Another technique is to increase the optical density of the coating to obscure the materials underneath and produce a white finish, by increasing the concentration of the white pigment titanium dioxide in the coating composition. Unfortunately the greater the concentration of titanium dioxide in the coating composition the poorer is the contrast of the mark produced in the coating by laser beam irradiation. Thus in general terms with such coatings increase of the titanium dioxide concentration in the coating increases the optical density and hence the whiteness of the coating but at the expense of reduced contrast and hence legibility in any marking produced in or on the coating by laser beam irradiation. A decrease of titanium dioxide concentration in the coating composition improves the laser marking effect, contrast and legibility but undesirably reduces the optical density and hence whiteness of the coating which can conventionally only be overcome by an undesirable increase in thickness of the coating.